To date the greatest interest from the point of view of creating coatings with defined physical and mechanical characteristics is taken in the so-called functional materials.
It is known that the electron-ray evaporation and subsequent condensation in vacuum of metallic and non-metallic materials is the most accurate method of construction of similar materials at atomic and molecular levels. By changing the precipitation temperature of concentration of phases being introduced and the rotation speed of the products being coated one could easily obtain coatings with introduced phase concentration gradients, microcellular or multi-layer coatings.
It is clear that for deposition of similar coatings in case of parts having a complex configuration, here included the gas turbine blades, suitable electron-ray sets are required. A series of vacuum apparatus designs is available for forming composite coatings.
Deposition of a 3-layer coating is carried out in a multi-chamber vacuum apparatus by moving the substrate from one chamber to another, one layer being precipitated in each of the chambers.
In the working chamber of the vacuum apparatus there are crucibles with evaporating materials which are placed in turns under the substrate which has to be provided with a protective coating.
In the working chamber of the vacuum apparatus the evaporators operate in turns, and the substrate and the mask plate parallel to it can turn and move independently.
The vacuum sets described above have a number of drawbacks:    a). The consecutive applying of just one layer in turn calls forth a lower output of the vacuum apparatus;    b). At the moment of going over to an another crucible there is a change in the evaporation rate of consecutive components which leads to non-uniformity of the structure in respect of its thickness and, as a result of it, to deterioration of physical and mechanical features on the whole;    c). The main drawback of the available technical solutions is the impossibility of coating the product from all sides. With the above-mentioned vacuum apparatus the protective layer is formed only on the part (article of product) side which is turned to the evaporator.
A number of vacuum apparatus are known to be developed for applying multi-component coatings to products with complex profiles (gas turbine blades) from all sides. Yet the design of these apparatus excludes the possibility of their being used for forming gradient and multi-layer coatings.
A detailed review of the electron-ray equipment designs used for coatication of protective coating is given in the prior art literature. The analysis of electron-ray equipment designs reveals the fact that the most universal industrial apparatus for deposition of protective composite coating when products with a complex shape are coated is the UE-175 (Y-175) apparatus designed at the NANU Electric Welding Institute named after E. O. Paton, which is comprehensively described in. The apparatus is designed mainly for forming protective anticorrosive coatings at gas turbines blades surfaces by way of electron-ray evaporation. The process of deposition of coating includes ion-plasmous cleaning and heating of blades placed into cartridges in a lock (preparatory) chamber with subsequent precipitation of the protective material evaporated from the crucibles to the surfaces of blades. The heating and evaporation of the material is carried out under the impact of electron rays. The apparatus constitutes a unit of vacuum chambers (the chambers for deposition of coatings and two pre-chambers) with mechanisms, devices and systems ensuring a half-uninterrupted manufacturing method. There are two cylindrical crucibles located in the chambers for deposition of coatings for carrying out evaporation of metal components out of them as well as three rectangular shuttle-type crucibles for evaporating metallic or ceramic components of the coating. The evaporation of the material from each crucible is carried out separately under the impact of electron rays coming from individually controlled electron guns.
Due to the fact that the products (blades) cool down in the process of their being moved from the pre-chamber to the chamber for deposition of coatings, there is one more gun located above the chamber for deposition of coatings which serves for heating up the blades before applying the coating. During the additional heating up the blades are screened from the crucibles (which are set in the evaporation mode) by turning screens. After the blades are heated up to the required temperature (which is monitored by means of pyrometers and thermocouples) the screens are opened and the coating is applied.
Unlike the above-mentioned technological solutions (U.S. Pat. No. 4,122,221 of Oct. 24, 1978; FRG Pat. No. 2813180 of Oct. 4, 1979), the apparatus allows to form not only multi-component coatings of the MeCrAlY-type, where Me-Co, Ni, Fe, but also composite coatings of the MeCrAlY-Me-O, Me-C-type.
In the process of operation of these apparatus at enterprises in Russia (NVO “Trud”, Samara; Litkarinsk Machine-Building Works, Moscow Region), Ukraine (SPB “Mashproyekt”, Mykolayiv; Southern Turbine Works “Zorya”, Mykolayiv), a number of design drawbacks have been detected. Preliminary heating of the blades in the pre-chambers proved to be improper. In consequence of permanent loading/unloading there is condensate accumulating from the air in the pre-chambers which thereafter causes formation of oxide films on the surfaces of the blades when they are heated. When thereafter the protective layer is applied, the presence of such a separating layer inevitably leads to peeling off of the coating in the process of operation of the blades.
In the process of evaporation of oxide, carbide or boride compositions from the “shuttle”-type crucible there are craters forming on the surface of the materials being evaporated which inevitably leads to changes in the speed of evaporation of these compositions and, as a result, the composite coatings of the MeCrAlY-Me-O, Me-C, Me-B type have a non-uniform chemical composition throughout the thickness and are not serviceable.
Therefore, a number of important modifications have been made to the design of the UE-175 apparatus, and the more recent versions of the apparatus (UE-187, UE-187 M apparatus) are provided with a crucible device which consists of 4 cylindrical crucibles arranged in a row. This type of the crucible device allows to ensure continuous feeding of the material being used to the evaporation area. Bars or up to 800 mm long billets of ceramics can be loaded to the crucibles. All guns are provided with electron ray scanning programmers. So, by choosing the appropriate scanning programme, one can ensure uniform evaporation of the components which are sublimated during electron ray heating without formation of any craters. The apparatus of this type are provided with automated technological process control systems. Therefore by choosing appropriate programs one can easily obtain composite disperse-reinforced or micro-layer coatings of corresponding MeCrAlY−MeO, MeC, MeB or MeCrAlY/MeCrAlY+MeO, MeC, MeB types; coatings with phase gradient along the thickness. The technology of applying such coatings is described in detail in. The industrial apparatus of the UE-187 M type designed at the NANU Electric Welding Institute named after E. O. Paton for coatication of two-layer and multi-layer heat-insulating coatings are used by US and German firms, in particular by the American firm “Pratt and Whitney”.
Nevertheless, despite wide potentialities offered by this equipment, the American firm “Pratt and Whitney” uses now a combined method of applying heat-reflecting coatings. The inner metal Ni(Co)CrAlYHfSi-layer is applied by plasma spraying, whereas the outer ceramic layer is applied by electron-ray deposition.
Such a technical solution is caused by the impossibility of introduction of a required amount of itrium, hafnium, silicon, zirconium into the inner metal layer by evaporation from one source.
In general, the crucible device with linear arrangement of the four cylindrical crucibles may be used for obtaining metal MeCrAlY coatings alloyed additionally with zirconium, hafnium or silicon. It can be achieved by means of independent evaporation of MeCrAlY-type alloys and refractory metals from autonomous sources (crucibles). Yet in case of a linear arrangement of the crucibles it is difficult to ensure a uniform distribution of components in the coating along the blade when, for example, the following technological scheme of evaporation is implemented: the MeCrAlY alloy—evaporation from the central crucible; alloying addition, hafnium—from the crucibles adjacent to the central crucible on its left and right. When simultaneous introduction of one more addition, for example, silicon, to the coating is required, the use of such a technological scheme becomes impossible at all as during evaporation of three different materials from three independent crucibles any chemical uniformity of the chemical composition is out of the question.
When the said technological scheme is used, it is impossible to precipitate two-layer heat-reflecting coatings of the MeCrAlYHfSi/MeO type during one single technological cycle as it would require to load preliminarily at least three crucibles with the components of the metal layer of the coating, and only after that use the same crucibles for precipitating the ceramic layer. So it has been proposed in to implement a new crucible device design with respect to the UE-175, UE-187 apparatus that are produced serially, which would allow to eliminate all the drawbacks described above. The crucible device is provided additionally with “shuttle”-type crucibles which are made in the form of semi-rings ensuring the maximum closeness to the central crucible. The said design of the crucible device allows to precipitate the MeCrAlY alloy from the central crucible, the Y, Hf, Si, Zr alloying additions from the “shuttle”-type crucibles, and the ceramic component from the other three cylindrical crucibles. In this case the Y, Hf, Si, Zr alloying components are placed in the crucibles in form of separate tablets (bricks, bars) geometrically with precise definition of their location along the perimeter of the crucibles. The mass of the Y, Hf, Si, Zr tablets (bars) and their geometrical allocation in the crucibles are defined so as to obtain the required concentration of the said elements in the MeCrAlYHfSiZr layer, and they depend also on the dimensions of the parts being coated.
The electron-ray gun which is used for evaporating the Y, Hf, Si, Zr alloying components is provided with a special electronic unit which allows to change under a given program the density of the electron ray depending on the perimeter of the surface of the crucibles which are loaded with the tablets (bars) of the Y, Hf, Si, Zr alloying components. So, by changing the density of the electron ray, the geometrical dimensions of the alloying components billets (bars) and their allocation in the crucibles, one can obtain the required concentration of the alloying additions in the coating throughout the perimeter of the products being provided with the protective coating.
Due to doping the MeCrAlY Y, Hf, Si, Zr matrix alloys and presence of disperse oxide additions in the composite micro-layers, the diffusion processes at the inter-layer boundaries become more complicated. Formation of layers on the basis of complex spinels of the 2Y2O3*Al2O3, 3Al2O3*2SiO2 type occurs in the process 2-2.5 times slower than it would take place under the same testing conditions in case of two-layer MeCrAlY/MeO coatings.
Industrial electron-ray apparatus of the UE-175, UE-187 type that are provided with such crucible devices ensure obtaining of practically the whole line of protective coatings, from the simplest one-layer coatings of the MeCrAlY type to two-layer MeCrAlYHfSiZr/Me type and three-layer MeCrAlYHfSiZr/MeCrAlYHfSiZr+MeO/ZrO2—Y2O3 type coatings, where MeO is the aluminium oxide or itrium oxide stabilized zirconium dioxide. In this case the composite MeCrAlYHfSiZr+MeO layer may be made in the form of alternating metal MeCrAlYHfSiZr and composite MeCrAlYHfSiZr+MeO layers, the thickness of the mono-micro-layer being from 0.5 to 1.2 μm. It is possible also to obtain coatings with components and compositions concentration gradient and so on.
It seems that the next revolutionary step in the creation of a new generation of gas turbine apparatus will be the development of blades made of materials on the basis of refractory metals and alloys that do not require cooling. Today, obtaining of alloys on the basis of refractory metals with high level of mechanical characteristics does not pose any problems. The main problem in respect of their use in the gas turbines manufacturing industry is the problem of effective protection of the alloys from catastrophic oxidation in the process of their operation over a long period of time (hundreds and thousands of hours). Diffusive silicide coatings, especially when modified with alloying elements such as boron, aluminium, titanium, chrome and others, are one of the main types of coatings used for protection of the refractory metals and their alloys from high-temperature oxidation. According to the data given in the prior art literature there are more than 100 industrial firms and research centers in USA that develop high-temperature protective coatings, almost half of which engages in creating heatproof coatings for refractory metals. At the same time it is mentioned that for operation at high temperatures (up to 1573-2003K) the most promising is deemed to be the use of intermetallides, and first of all silicides. Yet the research works carried out during the last three decades did not result in creating reliable silicide coatings, which could effectively protect products made of refractory metals and alloys over long periods of time under extreme conditions of operation.
The main methods of obtaining silicide coatings and the industrial equipment required for that are described in detail in the prior art literature; the following main methods of obtaining silicide coatings might be singled out:                1). Saturation from steam and gas mixtures containing silicon compounds, mostly haloid ones, with hydrogen or without it (gas-phase siliconizing);        2). Saturation in silicon vapour in vacuum (vacuum siliconizing);        3). Saturation from rare phase by electrolysis or without it (rare phase siliconizing);        4). Saturation in powder siliconeous mixtures with activators (gas-phase siliconizing in powder)        
It is pointed out that, as a rule, the vacuum silicide coatings have better technical characteristics compared to other methods. As a rule, the vacuum siliconizing is carried out from backfilling of high-clean silicon powder; furthermore, it can be carried out under conditions when the metal which is being saturated and the silicon are separated one from another and may be heated up to different temperatures. However, the vacuum siliconizing is a lengthy and costly process and is not notable for high output; there are also substantial limitations in respect of the overall dimensions and form of the parts.
There is one of the most important features out of the large variety of characteristics of the silicide coatings, due to which these coatings are mainly being developed, that needs to be examined, and it is the heat resistance. As the disilicides of the metals belonging to the sub-groups IV and VI have the highest heat resistance, it's exactly these phases that are usually used in coatings. Their behavior in the open air or in oxygen (at different pressures) in a large range of temperatures is rather well known. According to the data given in the prior art literature, the disilicides of the sub-groups IV and VI might be graded in the following order in respect of their resistance to open air oxidation: TiSi2, ZrSi2, NbSi2—resistant up to temperatures 1073-1373° K.; TaSi2—up to 1373-1673° K.; CrSi2, WSi2—up to 1673-1973° K.; MoSi2—up to 1973-2073° K.
Creation of coatings on the basis of complex silicide compositions doped additionally with boron, titanium and other elements is of extraordinary interest.
The operational reliability of the products having silicide coatings can be further increased by means of creating combined two-layer coatings of the silicide/oxide type (MeSi2/MeO).
However, the traditional methods of applying silicide coatings do not allow to obtain such combined two-layer or multi-layer coatings.
Electron-ray evaporation of metal and non-metal materials with their subsequent condensation in vacuum gives some chances in respect of obtaining such coatings.
However, the electron-ray apparatus designs considered above do not allow to carry out industrial precipitation of silicide coatings on parts by the following reasons.
As is well known, Si, Ti, Zr, Nb, W, Cr differ substantially in respect of vapor resiliency. So evaporation of compositions of the MeSi2 type from one source (crucible) is not possible.
In case of industrial electron-ray apparatus with multi-crucible evaporation and linear arrangement of crucibles there is a principal possibility of synthesizing similar compositions in vapor phase. Yet in this case there is substantial non-uniformity of the chemical composition of the silicide coating along the length of the product being coated, as for example in case of evaporation of Ti and Si from two linearly arranged crucibles. Precipitation of more complex silicide coatings from four linearly arranged crucibles is hardly imaginable at all.
Silicide coatings may be synthesized in electron-ray apparatus with multi-crucible evaporation where the crucibles are arranged in circle. One design of such an apparatus is described in the prior art literature.
The source materials in the form of bars or burnt billets were located in four copper water-cooled crucibles, 70 mm in diameter, which were arranged in circle. The bars or billets were placed on copper water-cooled rods connected with vertical feed mechanisms for replenishment of the material evaporated from the bath. Separated or mixed vapor flows were being precipitated on a revolving substrate made of 8-mm thick stainless steel in the form of a disk having 520 mm in diameter. The substrate speed was regulated in the range of 0.05 to 200 rpm.
During the technological cycle the prescribed substrate speed is maintained strictly constant with the help of a-single-phase thyristor unit ETO 1. Six electron-ray heaters with the capacity of 60 kW each are intended for evaporating the source materials and heating the substrate.
The apparatus is provided with control units for electron-ray heaters. The automatics system used provides for maintaining and regulating the necessary rate of evaporation of each component during the whole technological cycle and allows to carry out evaporation of materials in pulsed mode.
With the said apparatus one- or two-layer silicide coatings can be easily synthesized by changing the location of 4 crucibles arranged in circle, evaporating for example Ti and Si from two adjacent crucibles and Zr and Si from the two other crucibles. Using this technological scheme one could also easily form two-layer coatings of the MeSi2/MeO type. Yet the said apparatus allows to precipitate coatings only from one side of the product. Furthermore, it has a very low output as after applying the coating time is required for cooling the products down and loading the main processing chamber with the new group of products to be coated. In consequence of continual opening of the main processing chamber, condensate is being formed from the air at the chamber walls. When the products are heated, the moisture from the chamber walls is being condensed at their surface forming oxide films and this leads to pealing off of the coating applied, and this is inadmissible.
The apparatus that is the nearest to the apparatus claimed in respect of the technological main points is the one described in the Japan Patent No 54-18989 of Oct. 4, 1977, the scheme of which is shown in FIG. 1.
The apparatus is intended for applying coatings on products in form of fingers (rods) and has a number of drawbacks in respect of its use for precipitation of coatings on gas turbine blades:    a). Under such a scheme it is impossible to carry out locking of products as they are loaded when the working chamber is open, which has a negative impact on the adhesion feature of the layer sprayed onto the substrate.    b). The design of the cartridge with the products is so that the turning of each of the rods (13) with products stringed on it is transmitted through wheels (8a) rolling along the encircling ring (9) located externally relative to products (along the internal perimeter of the chamber), which is more complicated from the design point of view than providing the drive at the center.    c). When such a scheme is used, the problem of protecting the wheels (8a) and the encircling ring (9) from steam flow getting into them arises. Accumulation of condensate on the said parts brings about braking and, in some cases, even wheel spin when the wheels are rolling along the encircling ring (9). On the other hand, deformation of the II-form structure of the ring (9) is possible when it is overheated, which automatically excludes any uniform revolving of the products on their axis indispensable for obtaining uniform thickness coatings along the perimeter of the products being provided with protective coating.    d). As each cartridge rod has a certain diameter size in the cross-section perpendicular to the axis of the rod, the number of the rods in the cartridge is defined by the following relationship: the more rods are located around the cartridge, the larger the space around the vertical axis of the chamber not occupied by the products (see the hatched areas in FIG. 2 a,b,c).
In this case the most of the vapor obtained in the process of evaporating the alloy from the central crucible (see FIG. 1) is not used for forming the coating (does not get onto the surface being coated).